When you hear the words ‘blue supergiant’, you’d be forgiven for thinking of a fairy
tale creature.
But the reality is actually far more fantastical: elusive astrophysical bodies that are notoriously
difficult to see and understand but which hold the keys to the formation of stars and
galaxies.
And for the first time, we can now see what’s inside them.
In the classification of stars, temperature is what determines color.
Just like the hottest part of a flame is that blue part at the bottom, the hottest stars
in the universe are blue.
And blue supergiant stars—as their name would suggest—are huge, with masses anywhere
upwards of 10 times that of our Sun.
They’re also much brighter and much hotter than mid-size stars like the Sun, making them
some of the most visible and recognizable stars in the night sky even though they’re
actually quite rare.
They burn hot and fast, only lasting, oh around tens to hundreds of millions of years or so.
And that’s compared to our Sun’s expected lifetime of 10 billion years.
Because they flame out so fast, relatively speaking, that makes them difficult to study.
With the advent of advanced telescopes both on the ground and in space, we’ve been able
to observe their surfaces, but we still know relatively little about what goes on inside
them.
Something has to be happening to produce their incredibly intense luminosity—their brightness—which
seems to shimmer and twinkle.
And new research has produced some of the first models of the interior of these enigmatic
stars, telling us a little more about how all this showstopping behavior comes to be.
A cooperative research effort between Newcastle University in England and KU Leuven in Belgium
used existing photometric data gathered by NASA’s Kepler/K2 satellite and the Transiting
Exoplanet Survey Satellite or TESS.
These satellites’ original purpose was to scour the galaxy for exoplanets, or planets
beyond our solar system.
But to do that, they surveyed hundreds of thousands of the brightest stars near our
sun, building a super handy data set for scientists interested in looking at bright stars...data
that confirms their existing hypotheses—and simulations—of how these stars behave.
Because the satellites, with their very sensitive detectors, were looking out at the sky for
a long enough period of time, they let us see what we’ve never seen before: the patterns
of shimmering and twinkling on blue supergiants.
Or, to put it in a more ‘scientific’ way, the coherent pulsations...indicating that
the stars are being rippled by internal gravity waves.
These are not to be confused with gravitational waves.
Gravity waves are disturbances in some substance due to the restoring effect of gravity: Say
a substance was going in one direction because of some other force like wind traveling across
the ocean, but the water is also being forced down by gravity, which—in combination with
the water’s natural buoyancy and other physical factors—results in disturbances of the water
that we know as waves.
This happens in our oceans and our atmosphere all the time…but it’s a very different
phenomenon from gravitational waves, which are tiny perturbations in the fabric of spacetime
due to some huge cosmic collision.
Different things, confusingly similar name, and though they’re common in our own oceans,
gravity waves are also what’s happening inside blue supergiant stars!
In analyzing their data set, the research team observed the presence of an entire spectrum
of low-frequency gravity waves in blue supergiants.
These flow through the star and break at the surface, a lot like ocean waves breaking onto
a beach.
These frequencies also include standing waves, a type of wave that’s a lot like a seismic
wave from an earthquake.
This revelation means that we can now delve into blue supergiants using asteroseismology.
Studying these frequencies allows astrophysicists to work out the chemistry and physics that’s
likely going on inside these stars, including at their core.
These calculations can tell us how metal is being produced in the stars and how it moves
around inside them... which is very important because blue supergiants are like the metal
factories of the universe, producing most of the chemical elements on the periodic table
past helium.
But perhaps even more importantly, this new information gives us insight we’ve never
had before into how the lives of these stars might end.
When these incredibly energetic and short-lived stars die, they explode into supernovae and
form a black hole or a neutron star.
What’s ejected in that explosion and what’s left over afterwards has a significant impact
on the formation of other stars and on the evolution of the galaxy around it.
As the researchers put it, massive stars like blue supergiants, “determine the evolution
of the cosmos.”
The results of this research gives us new ways to look at these potential supernovae,
like using asteroseismology, to produce highly accurate models of their ages, what’s going
on in their core, their rotation, angular momentum transport, interior mixing—all
of which could tell us more about when and how they’ll die, and what they’ll put
out into the universe when they do…which could potentially inform our understanding
of how other galaxies and stars formed before them, too.
If you want even more galactic exploration, check out my other video here about the universe’s
first molecule.
Make sure to subscribe to Seeker for more explorations of the universe, and as always,
thanks for watching.